During the fall of 2022, an outbreak of measles occurred in Ohio, ultimately leading to 85 cases of a disease that officially had been eliminated from the United States two decades ago. A recent report by public health investigators describes the outbreak. Their most significant finding? The virus spread almost if not entirely among people who were unvaccinated or only partially vaccinated.
In a report published by the CDC on Friday in Morbidity and Mortality Weekly Report, researchers provide information about likely exposure locations, clinical outcomes of the infected people, and the vaccination status of those individuals. Their conclusion? According to the authors of the study, “Sustaining elimination of measles in the United States will require continued high 2-dose MMR vaccination coverage in all communities.” It’s a stark warning about a disease that some people may think only exists in the history books.
The story of measles in the United States is a public health success story. In the early part of the 20th century, roughly 6,000 measles-related deaths occurred per year in the United States. By the middle of that century, an estimated 3 to 4 million people were infected per year, leading to 400 to 500 deaths. But the development of a vaccine in the 1960s changed the landscape dramatically.
The measles vaccine, typically administered in combination with the mumps and rubella vaccines, consists of a weakened, or attenuated, form of the measles virus. After injection, the virus undergoes a limited amount of replication in the body, stimulating an immune response, but not causing disease. The recommended two-dose series is 97% effective at preventing infection. People who do get infected after vaccination tend to exhibit milder disease symptoms. It is thought that immunity lasts a lifetime.
As the vaccine was rolled out in the 1960s, the number of cases decreased precipitously. From 1960 to 1970, the number of cases dropped roughly 90%, from more than 400,000 per year to fewer than 50,000. In 2000, only 86 cases of measles were reported in the United States and the disease was declared eliminated from the country.
But elimination does not mean eradication. According to the World Health Organization, elimination means that continuous transmission of the virus has not occurred in a region for at least a year. Although continuous transmission of the virus no longer occurs in the U.S., measles remains common in other parts of the world. Typically, sporadic outbreaks of measles in the United States occur via an imported case and are relatively limited in scope. Someone gets infected in another country, travels to the U.S. and the virus is transmitted to other susceptible people.
So, what do we know about the outbreak in Ohio?
Between October and December 2022, 85 cases of measles were reported. For each infected individual, exposure most likely occurred at a health care facility, a childcare facility or at home. Genetic analysis of the virus from 65 people showed that everyone was infected with a genetically identical form of the virus. In other words, this outbreak originated from a single introduction of the virus into the community.
The authors note that four imported cases of measles had been reported in the area during the summer of 2022, all in people who had visited parts of East Africa where transmission of measles remains common. Presumably, one of these imported cases led to the subsequent outbreak in Ohio. However, analysis of viral genetic material from these individuals was not done, so the link between these individuals and the other cases remains speculative.
We do, however, know the link between the cases and vaccine status. All four of the individuals who traveled to East Africa were unvaccinated. Of the 85 other cases, 80 individuals were unvaccinated, four individuals had received only a single shot and the vaccination status of one person was not known, according to Columbus, OH officials.
This outbreak clearly shows the value of vaccines and the threat to public health of unvaccinated and partially vaccinated individuals. In this cluster of cases, 25 infected individuals were under the age of one and not yet eligible for the MMR vaccine. For other people in a community, including people who are pregnant or severely immunodeficient, the vaccine is not recommended. For these people, protection from measles can only occur when the vast majority of everyone else is vaccinated, thereby resulting in herd immunity. The presence of unvaccinated and partially vaccinated individuals disrupts this herd immunity. As a result, if enough members of a community are unvaccinated, then the most vulnerable members of the community face an increased risk.
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